The present invention relates to a method and an apparatus for subjecting a pipe member to bulging forming, and in particular, to bulge forming method and apparatus for carrying out bulge forming in the case where a raw member is a bend pipe.
Conventionally, some methods of forming a member as shown in FIG. 8, which has a different cross section in each portion and is bent as a whole such as a member for automobiles have been proposed. Japanese Patent Laid-open Publication (Kokai) No. 57-19114 discloses a method of manufacturing a member with an elbow. According to the method, a straight pipe is pressed so that the pipe is bent while applying an internal pressure thereto, and slightly subjecting the pipe to bulging process, thus forming an elbow of the pipe.
In order to form the member as described above, in general, processes as shown in FIG. 9 has been employed. Specifically, according to the processes, a straight pipe is subjected to bending, and then, the resultant pipe is pressed while being subjected to hydraulic bulge forming.
Japanese Patent Publication (Kokoku) No. 60-5126 discloses a method of manufacturing a branch pipe.
According to the method, a straight pipe with one bent portion is subjected to hydraulic bulge forming so that its center portion can be bulged, and an outside on the center of bulged bend pipe is subjected to burring, and subsequently, an opening having the same diameter as the original pipe is formed, thus a branch pipe being formed.
Further, Japanese Patent Publication (Kokai) No. 58-74221 proposes a technique of manufacturing a crankshaft. According to the technique, a U-shaped pipe having four bent portion with a large R is inserted in a die, and the pipe is subjected to hydraulic bulge forming so as to make sharp the R of the bent portion.
Furthermore, Japanese Patent Publication (Kokai) No. 58-199626 discloses a method in order to make small the R of an inside of the bend pipe. According to the method, a pipe is subjected to bending, and then, the pipe is subjected to hydraulic bulge forming after inserting the pipe into a die having an extremely small inside R. Whereby a bend pipe having a small curvature on its inside portion can be obtained.
Among the aforesaid conventional methods, according to a method of carrying out bending while applying an internal pressure; for example, the method disclosed in Japanese Patent Publication (Kokai) No. 58-19114, a principal purpose of applying the internal pressure is to prevent inward buckling caused in pressing. Therefore, this method does not positively carry out bulge forming, and is not suitable for forming a member as shown in FIG. 8, which must be partially bulged in cross section.
Except for Japanese Patent Publication (Kokai) No. 58-19114 which discloses the method of carrying out bending while applying an internal pressure, according to other techniques, a pipe is first subjected to bending by various forming methods, as shown in FIG. 9. At this point of time, a wall thickness of an outside portion of the member becomes considerably thin. FIG. 10 shows a wall thickness strain distribution on inside and outside portions of an electroseamed steel pipe (equivalent to STKM13B) when being subjected to rotary draw bending at an angle of 90.degree.. As seen from FIG. 10, the wall thickness of the inside portion becomes thicker than the initial one by bending. However, the wall thickness of the outside portion becomes 30% thinner than the initial one. For this reason, the steel pipe reaches the vicinity of forming limit by only bending. Therefore, it can be seen that there is a great danger of causing breaking. In addition, according to hydraulic bulge forming which is the next process, as shown in FIG. 11, the outside portion of the steel pipe is further bulged, and then, is subjected to upset forming. For this reason, the outside portion has a great danger of causing breaking more and more.
In particular, according to the technique disclosed in Japanese Patent Publication (Kokai) No. 58-74221, a cavity corresponding to the outside portion on a bent portion of the bend pipe is considerably large. For this reason, large bulge deformation is required. Moreover, in the case where the corner portion of the cavity is sharp, a wall thickness of a portion corresponding to the corner portion becomes extremely thin after forming. This results from the following reason. Specifically, when the pipe member comes into contact with the cavity, the member is suppressed from moving due to frictional effect, so that the wall thickness of the member is hard to be reduced. However, if the corner portion of the cavity is sharp, there is a considerable delay in timing when the member corresponding to the corner portion comes into contact with the cavity. For this reason, the wall thickness reduction speed does not become late.
Even if the corner R of bulging portion is small, for example, if a member is subjected to bulge forming such that it is bulged by the vicinity of breaking limit, or if a high strength member having low ductility is used, breaking is caused before the member is formed into the final product shape. As a result, a predetermined forming can not be carried out. Therefore, there is arisen a serious problem.
In order to solve the above problem, as seen from hydraulic bulge forming which is employed for manufacturing a T-shaped pipe of from a straight pipe, there has been proposed a technique in which a bulging allowable piston is arranged on a portion corresponding to the bulging, and the piston is retreated in accordance with the forming progress (see Japanese Patent Publication (Kokoku) No. 60-51209). However, according to the technique, the target member is limited to a straight pipe, and there is no reference to the matter that a pipe member is subjected to preforming such as bending, etc.